Comparison OPTICON Sky Navigator 70F700EQ vs OPTICON Pulsar 76F700

The design characterizes the general principle of the optical system of the telescope.

— Lens (refractors). As the name implies, in such telescopes, a system of lenses is responsible for building the image. Their main advantages are simplicity of design and use, as well as unpretentiousness to shaking, shock and adverse weather conditions (which facilitates outdoor use, including in the cold season). On the other hand, this scheme of operation requires the use of long tubes, which accordingly affects the dimensions of the structure, and the diameter of the lenses (see below) for refractors as a whole is noticeably smaller than for reflectors. In addition, lenses are subject to various distortions, in particular, chromatic aberrations, which lead to the appearance of colour halos and reduce image quality. However, modern telescopes often use various design tricks to neutralize these distortions. Refractors are well suited for observations of relatively close objects such as the Moon or planets, as well as panoramic observations at a relatively low power. In addition, this option is considered optimal for beginner astronomers, including children.

— Mirror (reflectors). In telescopes of this design, the role of the lens is played by a concave mirror, which provides the main magnification of the image. The simplest and mos...t popular reflex scheme — Newton's telescope — involves a combination of a concave main mirror with an additional flat one, which reflects the image into the eyepiece. There are other variations of reflectors, but they are much more complicated and expensive, and therefore they have not received distribution in amateur astronomy. Anyway, telescopes of this type, being simpler, cheaper and more compact than refractors, have larger lenses and are less prone to distortion, which makes it possible to obtain high-quality images of fairly distant objects. Their main disadvantage is delicacy and difficulty in handling. Thus, mirrors are sensitive to shocks and shocks, the optics need to be adjusted from time to time, and before starting observation, it is necessary to wait for temperature equilibrium — otherwise the difference in air temperatures in the tube and outside will lead to a loss of image clarity (the same "haze" effect that can be seen above hot asphalt on a summer day). Also note that most reflectors give distortions at the edges of the image (the so-called "coma"), which narrows the actual field of view and makes it difficult to use them for astrophotography. However, in many models this drawback has been corrected, in others it is possible to use corrective lenses and other similar accessories, due to which reflectors are still the most popular option among astrophotographers.

— Mirror lens. Such telescopes, in fact, are mirror models (see above), designed according to specific schemes and supplemented with corrective lenses to eliminate various distortions. Thanks to this, it becomes possible to further improve the quality of the "picture" compared to classical refractors, while at the same time retaining their main advantages — primarily compactness and relatively low cost. Among the mirror-lens models, there are also several different systems. Thus, Schmidt-Cassegrain systems are compact, inexpensive, and not as sensitive to small shocks as classical Newtonian reflectors; and the Maksutov systems (Maksutov-Cassegrain for close objects and Maksutov-Newton for distant ones) are somewhat more expensive, but are considered more advanced.

Telescope objective diameter; this parameter is also called "aperture". In refractor models (see "Design"), it corresponds to the diameter of the entrance lens, in models with a mirror (see ibid.), it corresponds to the diameter of the main mirror. Anyway, the larger the aperture, the more light enters the lens, the higher (ceteris paribus) the aperture ratio of the telescope and its magnification indicators (see below), and the better it is suitable for working with small, dim or distant astronomical objects (primarily photographing them). On the other hand, with the same type of construction, a larger lens is more expensive. Therefore, when choosing for this parameter, it is worth proceeding from the real needs and features of the application. For example, if you do not plan to observe and shoot remote (“deep-sky”) objects, there is no need to chase high aperture. In addition, do not forget that the actual image quality depends on many other indicators.

Designing and manufacturing large lenses is not an easy and expensive task, but mirrors can be made quite large without a significant increase in cost. Therefore, consumer-grade refracting telescopes are practically not equipped with lenses with a diameter of more than 150 mm, but among reflector-type instruments, indicators of 100-150 mm correspond to the average level, while in the most advanced models this figure can exceed 400 mm.

The luminosity of a telescope characterizes the total amount of light "captured" by the system and transmitted to the observer's eye. In terms of numbers, aperture is the ratio between the diameter of the lens and the focal length (see above): for example, for a system with an aperture of 100 mm and a focal length of 1000 mm, the aperture will be 100/1000 = 1/10. This indicator is also called "relative aperture".

When choosing according to aperture ratio, it is necessary first of all to take into account for what purposes the telescope is planned to be used. A large relative aperture is very convenient for astrophotography, because allows a large amount of light to pass through and allows you to work with faster shutter speeds. But for visual observations, high aperture is not required — on the contrary, longer-focus (and, accordingly, less aperture) telescopes have a lower level of aberrations and allow the use of more convenient eyepieces for observation. Also note that a large aperture requires the use of large lenses, which accordingly affects the dimensions, weight and price of the telescope.

This item indicates the eyepieces included in the standard scope of delivery of the telescope, or rather, the focal lengths of these eyepieces.

Having these data and knowing the focal length of the telescope (see above), it is possible to determine the magnifications that the device can produce out of the box. For a telescope without Barlow lenses (see below) and other additional elements of a similar purpose, the magnification will be equal to the focal length of the objective divided by the focal length of the eyepiece. For example, a 1000 mm optic equipped with 5 and 10 mm "eyes" will be able to give magnifications of 1000/5=200x and 1000/10=100x.

In the absence of a suitable eyepiece in the kit, it can usually be purchased separately.

The magnification of the Barlow lens supplied with the telescope.

Such a device (usually, it is made removable) is a diverging lens or lens system installed in front of the eyepiece. In fact, the Barlow lens increases the focal length of the telescope, providing a greater degree of magnification (and a smaller angle of view) with the same eyepiece. In this case, the magnification factor with a lens can be calculated by multiplying the “native” magnification with a given eyepiece by the magnification of the lens itself: for example, if a telescope with a 10 mm eyepiece provided a magnification of 100x, then when installing a 3x Barlow lens, this figure will be 100x3=300x. Of course, the same effect can be achieved with an eyepiece with a reduced focal length. However, firstly, such an eyepiece may not always be available for purchase; secondly, one Barlow lens can be used with all eyepieces suitable for the telescope, expanding the arsenal of available magnifications. This possibility is especially convenient in those cases when the observer needs an extensive set of options for the degree of magnification. For example, a set of 4 eyepieces and one Barlow lens provides 8 magnification options, while working with such a set is more convenient than with 8 separate eyepieces.

The total weight of the telescope assembly includes the mount and tripod.

Light weight is convenient primarily for "marching" use and frequent movements from place to place. However, the downside of this is modest performance, high cost, and sometimes both. In addition, a lighter stand smooths out shocks and vibrations worse, which may be important in some situations (for example, if the device is installed near a railway where freight trains often pass).